1. Field of the Invention
The present invention relates to an electric power steering system of the type wherein an electric motor generates an assisting power to rotate a steering shaft of a motor vehicle.
2. Discussion of the Prior Art
There have been proposed many types of electric power steering systems which utilize an electric motor as a means for generating an assisting power to rotate a steering shaft. In the proposed systems, a battery is used as a power supply not only to a conventional electric system for the vehicle, but also to the power assisting electric motor. An alternator for charging the battery is driven by the vehicle engine, and thus, the electric power generated by the alternator largely varies depending upon the rotational speed of the engine. To avoid this, the voltage generated by the alternator is controlled by a voltage regulator in such a manner as to maintain the battery voltage constant, e.g., 12 volts.
However, electric motors presently available are incapable of generating a torque required for power assist of the steering system when they are driven under such a low voltage. This makes the practical use of any of the proposed electric power steering systems difficult. Although for a solution to this problem, it may be conceived to permit a sufficient electric current to flow across a rotor winding of the motor by thickening the rotor winding, such would necessarily result in enlarging the electric motor to the extent that the same cannot be practically used.
Further, in order to obtain a required torque for the power assist, a large electric current must be applied to the power assisting motor. This causes the control of the electric current applied to the motor to be difficult and also leads to a large controller as a result of using large capacity control elements. In addition, the control of such a large electric current by the large capacity control elements involves a great energy loss due to a large quantity of heat generated, thereby bringing about useless consumption of the vehicle engine power.